Antonym (Most opposite meaning) of the word CELESTIAL is

Concept

An antonym is the word whose meaning is most opposite to the given word. "Celestial" is an adjective meaning relating to the sky or the heavens — divine, heavenly, otherworldly. So its antonym must name what belongs to the ground/earth, not to the heavens.

Application

CELESTIAL = of the heavens / sky. The opposite domain is the earth / worldly realm. The choice that means "belonging to the earth, worldly, terrestrial" is the correct antonym — that is "Earthly."

Contrast with the near-misses

• "Heavenly" means of or like heaven — a synonym of celestial, not its opposite.

• "Divine" means godlike / sacred — again a synonym of the heavenly sense, not an antonym.

• "Supernatural" means beyond the natural / otherworldly — close in sense to celestial, so it is not the opposite either.

Only "Earthly" reverses the heavenly domain, so it is the most opposite in meaning. (Terrestrial / worldly / mundane are the standard dictionary antonyms of celestial.)